1. Field of the Invention
The present disclosure relates to a technique for providing a starting circuit suitable for a higher frequency in a discharge lamp lighting circuit.
2. Description of the Related Art
A known lighting circuit for a discharge lamp such as a metal halide lamp has a configuration which comprises a DC power source circuit formed as a DC-DC converter, a DC-AC converter circuit, and a starting circuit. For example, a DC voltage supplied from a battery is converted to a desired voltage by the DC power source circuit, the converted voltage is further converted to an AC output by the DC-AC converter circuit which is in a subsequent stage, and a starting signal (a so-called starter pulse) is superimposed on the AC output and then supplied to the discharge lamp (see, e.g., Japanese Patent Document JP-A-7-142182).
In such a configuration which conducts two steps of voltage conversion, which are DC voltage conversion and DC-AC conversion, when the circuit scale is large, the configuration is not suitable for miniaturization. Therefore, a configuration is used in which an output that is boosted by one step voltage conversion in a DC-AC converter circuit is supplied to a discharge lamp (see, e.g., Japanese Patent Document JP-A-7-169583).
In such conventional configurations, the starting circuits for a discharge lamp have a problem with the adaptability to a higher frequency.
When a secondary winding of a transformer (a so-called starter transformer) constituting a starting circuit is connected to a discharge lamp in series, and the output frequency of a DC-AC converter circuit being increased, for example, a large loss is produced in the transformer, thereby causing the lowering of efficiency. The increase of an operating frequency, which is the driving frequency of switching elements constituting a DC-AC converter circuit, is essential for miniaturizing the circuit, and hence the power loss in the starting circuit and the like must be suppressed to a low level as far as possible. When the voltage of the power supply to the starting circuit is low, problems of increase in cost and impeding of miniaturization may occur, such as those that circuit elements must have higher ratings in accordance with the increased current level, and the circuit scale becomes large.